Okay, so this is the first discussion that I'm joining so bear with me....
The word 'Druid' is commonly thought to be of Indo-European origin...'Dru' being oak, and 'wid' meaning to know. The Druids of Gaul were said to venerate the oak, especially where mistletoe was to be found growing upon it (a rare occurance), however the mistletoe is not native to Ireland and was only introduced some time in the last century. In fact it is known that Irish Druids reveared the Ash, Hazel and Rowan more than they did the oak...
The Druids used trees in their calender, being made up of 13 months each being asociated with a particular tree and an extra day. The Ogham (a form of writing, and latter of divining) also used tree and plant symoblism. At the center of the eight major fesivals of the year, Druids celebated (and still do) the same festivals as modern day Wiccans, was a massive fire burning the nine trees sacred to the Celtic people.
Among the four trees already metioned (Ash, oak, hazel and rowan) the druids also reveared the Alder, Yew, Poplar, Holly, Willow, Birch, Hawthorn, Honey-suckle, Mistletoe amongst others. All of these plants were accorded magical powers and used in ceremonies to symoblise diferent things and some of this symoblism still survives as folk tales to this day e.g. A marriage ceremony performed under an oak protected the couple from harm, where in parts of England this continued until the 18th century when it was outlawed as a pagan practise and newly weds still some times run for the nearest oak tree as soon as the leave the church.
Trees in general were seen as divine, as in many other cultures, because of their long life spans, and as obvious centers of natural reverence. Plants took on magical atributes based on it's physical and psychological ones, thus the oak who's wood is solid and strong becomes a protective magical infulence, it is also linked with thunder and storms because of the frequncey of lightning strikes on oak trees. Mistletoe berries are thought to represent seman thus is an obvious candiate as a fertility symbol.
That's about all I can remember sat at my computer, I hope I was of some help. I'll post more if I remember anything else of interest, or am asked.
Cleddau Brenin